Heat flow and gas hydrates on the continental margin of India

DE-NT0005669

Last Reviewed 12/15/2011

Goal

The goals of this project are to construct maps of apparent and residual heat flow through the western continental margin of India and to investigate the relationship of residual heat flow anomalies to fluid flow and gas hydrate distribution in the subsurface.

Performer

Oregon State University, College of Oceanic and Atmospheric Science, Corvallis, OR 97331

Map of the four regions sampled during NGHP Expedition 01

Background

Gas hydrate distribution in sediments depends on methane supply, which in turn depends on fluid flow. When drilling data are available to calibrate seismic observations of the base of the gas hydrate stability zone (GHSZ), the seismic observations can be used to map variations in apparent heat flow. Anomalies in apparent heat flow that cannot be attributed to other factors, like variations in thermal conductivity or topography, can be interpreted to result from subsurface fluid flow.

This project will integrate Bottom Simulating Reflection (BSR) data, high-quality in situ temperature measurements, and chemical analysis data from core samples obtained during the spring 2006 India expedition to map heat flow on the continental margin and to relate the apparent heat flow to fluid flow. Models of fluid flow will be used to estimate how much methane-rich fluid has potentially passed through a given sediment package and to predict the distribution of gas hydrates in the study areas.

Impact of this research

This project will contribute to our understanding of the impact of topography and stratigraphy on heat, and by proxy, fluid flow in continental margin sediments. The results will have broad applicability to understanding the factors that influence the distribution of gas hydrates and thus will contribute to the development of predictive models.

Accomplishments
OSU has generated 2-D models to depict regional variations in heat flow for the KG Basin and Andaman Sea. Initial results showed that heat flow values at any given depth, as well as the rate at which heat flow increases with depth, are on the order of 2.25 times lower in the Andaman Basin than those for the KG Basin. While the analysis showed that heat flow in these basins is influenced by topographic variations, other processes also appear to influence heat flow as there were strong variations in heat flow even where no topographic variation exists. In addition, where topography does affect heat flow, the theoretically modeled heat flow variations were smaller than those indicated by the apparent heat flow maps. Core samples from both sites indicate a lower thermal conductivity in the Andaman Basin, which explains the lower heat flow values. However, the thermal conductivity values observed were not sufficient to explain the large degree of heat flow variance between the two sites. These results suggested the need for expanded modeling to account for more complex geometries.

OSU revised apparent heat flow maps of the Krishna-Godavari and Andaman basins. By removing poorly constrained portions of the map and anomalies induced by seismic survey registration errors. Heat flow profiles across the Mahanadi basin were also constructed. Additionally, heat flows from various continental margin environments worldwide were compiled and compared to NGHP Expedition 01 BSR-derived heat flow to see if the increase in apparent heat flow observed in all three study sites is a common feature of continental margins. These findings have been compiled in a manuscript submitted to the journal Marine Geophysical Researches. The manuscript will be made publically available once the publication has been released.

OSU has modeled the effects of sedimentation on the temperature vs. depth profile in the Andaman Sea. At this site, the temperature gradient of 19°C/km is surprisingly low, leading to a very thick region in which gas hydrate is stable. Both anomalously old lithosphere and high sedimentation rates are possible explanations for the low apparent heat flow. Multiple modeling scenarios were constructed in which basement age, sedimentation rates, and sediment thickness were varied in an attempt to reconstruct the low apparent heat flow. Preliminary findings suggest that the low apparent heat flow found in the Andaman Sea would require very rapid sedimentation rates on the order of 2000 cm/ky (centimeter per 1000 years), which are not commonly found in deep marine environments. Sedimentation rates of this magnitude may have previously existed and when these rates are modeled between 3 to 2 Ma (million years) ago along with measured rates of 5.7 cm/ky for the past two million years, the model shows agreement with the observed low temperature gradients. However, one would expect to see major lithologic differences in the sediments as a result of these dramatic changes in sedimentation rates. Although no lithologic changes are reported, the porosity profile is suggestive of changes in sedimentation rates.

For the KG Basin, researchers have explored the degree to which spatial variations in the heat flow/water depth relationship can be explained by variations in local stratigraphy and structure. Sedimentation rates of 300 cm/ky are necessary to recreate low heat flow region (~25 mW/m2); 12 times faster than today's estimated sedimentation rates of 25 cm/ky. Seismic data reveals significant faulting near the edge of the KG Basin. These faults represent potential pathways for advective heat transport from the basin center to the basin edge, resulting the observed low apparent heat flow in the basin center and high apparent heat flow at the basin rim.

Current Status (December 2011)
This project has been completed and the Final Scientific/Technical Report has been submitted and approved (see "Additional Information" below).

Additional Information
In addition to the information provided here, a full listing of project related publications and presentations as well as a listing of funded students can be found in the Methane Hydrate Program Bibliography [PDF].